[HTC Sensation] heavy_metal_man:
Answers In a neat format now im at the pc :P

1) Does it help to just root and install something to make it go faster? (so all my apps, files and settings stay)

Cyanogenmod is not a kernel, cyanogenmod is an open source aosp built version of the android system :) they have there own kernels though;) It will be installed when you install the rom. as a rule of thumb never install a sense kernel to a aosp rom and vise versa.

2) Better to root the phone and install another Android version?

If you want to jump to performance and jellybean then -->this<-- is the rom for you!

3.1) Cyanogenmod or something else?

Cyanogenmod, but from the devs at xda ;)

3.2) Why?

I would recommend the rom above as it has all the features of cyanogenmod but with the developers added support as well!

3.3) And where can I find a tutorial to install this for my phone?

This guide is no longer relivant to your device as you undoubtedly have the updated hboot. This is what you need to remove the security from your new hboot :) once that is done i recommend using this guide to upgrade your firmware.This is to ensure rom compatibility :) once this is done install this recovery via fastboot. After that Simply follow the install instructions from the first posts of the rom thread i have linked above :)

4.1) What are the pros and cons of Cyanogenmod (or another one) vs the default?

Yes, for your needs :)

4.2) And what are the pros and cons of rooting/installing another Android version in general?

Any android rom running jellbean will have Project butter inside ;) Including the rom linked above.

4.3) Is there some noob/beginners article on this?

yes, check here This should be all you will ever need :)

4.4) Can I still use Google Play?

Yes you can still use google play :) Gapps stands for Google Apps. What happend was a few years ago cyanogenmod issued there roms with gapps pre-installed and google issued them with a cease and desist order stating that only they, and agreed upon companies could distribute google apps. cyanogenmod stopped for a little bit as they considered their position. Eventually it came about that having gooogle apps as a flashable zip was the answer, as it was not directly distributed with cyanogenmod. from then on all aosp/aokp roms that i am aware of followed suit and started using gapps!

5.1) If I install another version I've to reset my phone?

nope :P

5.2) I'd like to stick with my apps, files and settings. Is this possible?

6) If question 5.2 isn't possible, not a very big deal; I can re-install apps, reset settings and backup and restore my files (photos and contacts). But there's only one app I don't know how to backup: Smurf Village (for my girlfriend). She'll kill me if I lose the save game. Does somebody know how I can backup and restore that game?

You may be in luck pal :) since you are on ics this solution should be applicable to you ^^ i would use the droid explorer application first from your windows pc and see how you get on, before you start the s-off process :)

7) Does a custom Android version auto update? So I don't have to update my phone manual time by time?

yes cyanogenmod does, as does bruces rom i believe. (linked above)

8) I can set the language to Dutch in all custom roms?

allowing the rom has support for the language yes :) (as far as i know most aosp roms will have support for most languages :))